The effects of magnetic fields on observational signatures of atmospheric escape in exoplanets: Double tail structures

CAROLAN, S.; VIDOTTO, A.A.; HAZRA, G.; VILLARREAL D'ANGELO, C.; KUBYSHKINA, D.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2021 vol. 508 p. 6001 - 6012

0035-8711

Using 3D radiative MHD simulations and Lyman-α transit calculations, we investigate the effect of magnetic fields on the observational signatures of atmospheric escape in exoplanets. Using the same stellar wind, we vary the planet´s dipole field strength (Bp) from 0 to 10G. For Bp < 3G, the structure of the escaping atmosphere begins to break away from a comet-like tail following the planet (Bp = 0), as we see more absorbing material above and below the orbital plane. For Bp ≥ 3G, we find a ´dead-zone´ around the equator, where low velocity material is trapped in the closed magnetic field lines. The dead-zone separates two polar outflows where absorbing material escapes along open field lines, leading to a double tail structure, above and below the orbital plane. We demonstrate that atmospheric escape in magnetized planets occurs through polar outflows, as opposed to the predominantly night-side escape in non-magnetized models. We find a small increase in escape rate with Bp, though this should not affect the time-scale of atmospheric loss. As the size of the dead-zone increases with Bp, so does the line centre absorption in Lyman-α, as more low-velocity neutral hydrogen covers the stellar disc during transit. For Bp < 3G the absorption in the blue wing decreases, as the escaping atmosphere is less funnelled along the line of sight by the stellar wind. In the red wing (and for Bp > 3G in the blue wing) the absorption increases caused by the growing volume of the magnetosphere. Finally we show that transits below and above the mid-disc differ caused by the asymmetry of the double tail structure.